Several methods are already known for synthesizing a N,N-dialkyl-hydroxylamine; in particular nitrones can be reduced either to disubstituted hydroxylamines by means of lithium aluminum hydride or by means of potassium boro-hydride or by hydrogenation on platinum black [Coll. Czech. Chem. Comm. 20, 202 (1955), JACS 79, 5739 (1957); 78, 6208 (1956); Gazz. Chim. Ital. 51, II, 306 (1921)]. The pyrolysis of trialkylamine oxides, known the Cope reaction, is useful for the synthesis of N,N-dialkyl-hydroxylamines as well.
Should the amine oxide have more than one alkyl group capable of forming an olefin, a mixture of hydroxylamines is obtained [Org. Synthesis Coll. Vol. IV; 612 (1963)].
The N,N-dialkyl-hydroxylamines may also be prepared either by reaction of compounds containing an N-O bond, by letting said compounds react with organometallic compounds [J. Chem. Soc. 119, 251 (1921)] or by alkylation of hydroxylamines or N-alkylhydroxylamines with alkyl halogenides [J. Org. Chem. 28, 1068 (1963); U.S. Pat. No. 3,491,151; C.A. 72, 132,130f (1970)]. It is also known that secondary amines, when treated with hydrogen peroxide or with acylperoxides, give rise to N,N-dialkyl-hydroxylamines .vertline.Chem. Ber. 65, 1799 (1932); Arch. Pharm. 299, 166 (1966); JACS 72, 2280 (1950); J. Chem. Soc. 3144 (1963).vertline.. The reaction is of a general type and can be used with primary amines as well. The modalities followed till now proved to be, however, extremely unsatisfactory, owing to the low yield in the desired product. Moreover the oxidation of the carbon atoms in alpha position, with respect to nitrogen, gave rise to a complex mixture of products. The oxidation of secondary amines with hydrogen peroxide was carried out in the presence of a reaction promoter as well, in particular in the presence of an ester of formic acid (German Patent No. 1.004.191) or in the presence of a usual catalyst, containing Mo, W and the like (Belgian patent No. 615,736). In any case the yields are low, whereas the reaction of decomposition of hydrogen peroxide clearly prevails over the reaction giving rise to hydroxylamine. Also the above mentioned preparation processes of N,N-dialkylamines, which are not based on oxidation by means of hydrogen peroxide-therefor, are characterized by use of expensive reactants, by the handling of rather unstable compounds, by formation of a large number of by-products, and by difficulty in separating the useful product.
It has now, surprisingly, been found that the preparation of N,N-dialkylhydroxylamine (in particular of N,N-diethyl-hydroxylamine) can be very much improved by carrying out the oxidation of the corresponding amine by means of hydrogen peroxide in the presence of a particular catalyst.